In acoustic imaging, resolution refers to the ability of an acoustic imaging apparatus to differentiate between two objects that are spatially or temporally separated from each other. It is known that acoustic image quality (or resolution) mainly depends on four factors: axial, lateral, elevation, and temporal resolutions. See, e.g., M. Peikari et al., “Effects of Ultrasound Section-Thickness on Brachytherapy Needle Tip Localization Error,” 14TH INTERNATIONAL CONFERENCE ON MEDICAL IMAGE COMPUTING AND COMPUTER-ASSISTED INTERVENTION (MICCAI), 2011. FIG. 1A illustrates the relationship between axial, lateral, and elevation directions in an acoustic imaging apparatus. As illustrated in FIG. 1B, axial resolution depends on the ability of the acoustic imaging apparatus to distinguish two objects lying at different depths, parallel to the direction of the acoustic beam. Axial resolution depends on the acoustic pulse length (in general, it is roughly equal to one half of the pulse length). As illustrated in FIG. 1C, lateral resolution refers to the ability of the acoustic imaging apparatus to distinguish between two objects lying beside one another and perpendicular to the acoustic beam. The lateral resolution depends on the distance between adjacent element beams (i.e., the beam width). As illustrated in FIG. 1D, the elevation resolution is the acoustic slice thickness. Finally, the temporal resolution refers to the frame rate of the acoustic imaging apparatus.
In general, an acoustic imaging apparatus has several scales for adjusting such image resolution parameters. When the acoustic beam is emitted from the acoustic transducer(s), it has a narrow width, but the width is not a uniform width at all depths of the image. Instead, it converges at the point of the “focal zone,” and becomes wider as it proceeds distally. The focal zone is the thinnest part of the acoustic beam.
As can be seen for example in FIG. 2, optimization of the depth and beam focus setting makes a substantial difference in the quality of the acoustic images (see T. K. Chen et al., “A Real-Time Freehand Ultrasound Calibration System with Automatic Accuracy Feedback and Control,” ULTRASOUND IN MED. & BIOL., Vol. 35, No. 1, pp. 79-93, (2009)). In particular, FIG. 2 illustrates the effects of axial resolution and lateral resolution on the appearance of the cross-section of a nylon wire in an acoustic image. The axial resolution does not change with the image depth; however, the lateral width of the dot is directly proportional to the lateral resolution.
Typically, in ultrasound-guided medical procedures such as brachytherapy the physician visually locates the current position of the needle tip (or catheter tip) on a display screen or monitor. Furthermore, a physician may visually locate the current position of the needle on a display screen or monitor when performing other medical procedures, such as biopsy procedures for the prostate and liver. The needle tip appears as bright spot in the image on the display screen. Accurate localization of the needle is a challenging problem due to existing artifacts in acoustic images. It has been shown that acoustic images may contain a number of artifacts caused by both within-plane (axial and lateral beam axes) and orthogonal-to-the-plane (elevation beam width) acoustic beam formation.
Proper adjustment of imaging parameters can help reduce artifacts and as a result, reduce error in localization of the needle tip. In particular, in order to reduce such image artifacts the focal zone (narrowest part of the acoustic beam) should be adjusted to focus the zone at the exact location of the needle tip.
Currently, such adjustments are manually controlled by the operator of the acoustic imaging apparatus. For example, during a brachytherapy procedure, the clinical physician visually identifies the approximate location of the needle tip and manually adjusts the gain, beam focus and depth of the acoustic imaging apparatus, accordingly.
However, this can be quite challenging since the non-uniform beam formation along the acoustic slice (lateral resolution) may cause errors in accurate localization of the needle tip. This can become quite tedious, time consuming, and even produce erroneous results due to human eye error.
Accordingly, it would be desirable to automatically adjust the focus of an acoustic beam employed by an acoustic imaging system, and the resulting acoustic images, to match the position of a surgical implement, e.g., the tip of a needle, during a medical procedure without the need for manual adjustment by the operator.
In one aspect of the invention, a method comprises: receiving location data from an electromagnetic tracking apparatus, the location data indicating a location of an object being tracked by the electromagnetic tracking apparatus within a region of interest within biological tissue, the data indicating the location of the object with respect to an electromagnetic tracking coordinate frame employed by the electromagnetic tracking apparatus; employing a processor to transform the location data from the electromagnetic tracking coordinate frame to an acoustic image coordinate frame employed by an acoustic imaging apparatus which is configured to employ an acoustic beam to generate acoustic image data for displaying acoustic images of the region of interest, where the acoustic image coordinate frame is different from the electromagnetic tracking coordinate frame; using the transformed data to generate at least one command for the acoustic imaging apparatus to cause the acoustic imaging apparatus to adjust at least one image resolution parameter employed by the acoustic imaging apparatus to cause the acoustic images to be focused at the location of the object; and communicating the at least one command over a communication channel to the acoustic imaging apparatus.
In some embodiments, the object is a surgical implement and the location data indicates a location of a tip of the surgical implement.
In some versions of these embodiments, the surgical implement is a needle having an electromagnetic sensor disposed at the tip.
In some embodiments, the at least one image resolution parameter includes at least one of a focal length of the acoustic beam, an image depth of the acoustic beam, a time gain compensation, and a zoom window, employed by the acoustic imaging apparatus.
In some versions of these embodiments, the image resolution parameter includes at least one of the focal length and the image depth of the acoustic beam.
In some embodiments, the communication channel comprises an Internet connection.
In some embodiments, transforming the location data from the electromagnetic tracking coordinate frame to the acoustic image coordinate frame comprises using a first transform which transforms the location data from the electromagnetic tracking coordinate frame to a reference coordinate frame and a second transform which transforms the location data from the reference coordinate frame to the acoustic image coordinate frame.
In some embodiments, the method further comprises performing a calibration procedure to produce calibration parameters, and employing the calibration parameters to transform the location data from the electromagnetic tracking coordinate frame to the acoustic image coordinate frame.
In another aspect of the invention, an apparatus comprises: an input configured to receive location data from an electromagnetic tracking apparatus, the location data indicating a location of an object being tracked by the electromagnetic tracking apparatus within a region of interest within biological tissue, the data indicating the location of the object with respect to an electromagnetic tracking coordinate frame employed by the electromagnetic tracking apparatus; a processor configured to transform the location data from the electromagnetic tracking coordinate frame to an acoustic image coordinate frame employed by an acoustic imaging apparatus which is configured to employ an acoustic beam to generate acoustic image data for displaying acoustic images of the region of interest, where the acoustic image coordinate frame is different from the electromagnetic tracking coordinate frame, and to use the transformed data to generate at least one command for the acoustic imaging apparatus to cause the acoustic imaging apparatus to adjust at least one image resolution parameter employed by the acoustic imaging apparatus to cause the acoustic images to be focused at the location of the object; and an output configured to communicate the at least one command over a communication channel to the acoustic imaging apparatus.
In some embodiments, the input and the output comprise an Internet connection.
In some embodiments, the object is a surgical implement and the location data indicates a location of a tip of the surgical implement.
In some versions of these embodiments, the surgical implement is a needle having an electromagnetic sensor disposed at the tip.
In some embodiments, the at least image resolution parameter includes at least one of a focal length of the acoustic beam, an image depth of the acoustic beam, a time gain compensation, and a zoom window, employed by the acoustic imaging apparatus.
In some versions of these embodiments, the at least one image resolution parameter includes at least one of the focal length and the image depth of the acoustic beam.
In some embodiments, transforming the location data from the electromagnetic tracking coordinate frame to the acoustic image coordinate frame comprises using a first transform which transforms the location data from the electromagnetic tracking coordinate frame to a reference coordinate frame and a second transform which transforms the location data from the reference coordinate frame to the acoustic image coordinate frame
In yet another aspect of the invention, a method comprises: tracking a location of an object while the object is disposed within a region of interest within biological tissue, the location of the object being determined with respect to a tracking coordinate frame; generating acoustic images of the region of interest, the acoustic images being generated with respect to an acoustic image coordinate frame which is different from the tracking coordinate frame; employing a processor to transform the location of the object from the tracking coordinate frame to the acoustic image coordinate frame; and automatically adjusting at least one image resolution parameter of the acoustic images in response to the location of the object with respect to the acoustic image coordinate frame.
In some embodiments, an acoustic imaging apparatus generates the acoustic images of the region of interest, and wherein automatically adjusting at least one image resolution parameter of the acoustic images in response to the location of the object comprises receiving at least one command at the acoustic imaging apparatus via a communication channel to which the acoustic imaging apparatus is connected.
In some versions of these embodiments, the at least one command causes the acoustic imaging apparatus to adjust at least one of a focal length, an image depth, a time gain compensation, and a zoom window, employed by the acoustic imaging apparatus.
In some versions of these embodiments, the communication channel employs an Ethernet protocol.
In some embodiments, transforming the location of the object from the tracking coordinate frame to the acoustic image coordinate frame comprises using a first transform which transforms the location data from the tracking coordinate frame to a reference coordinate frame and a second transform which transforms the location data from the reference coordinate frame to the acoustic image coordinate frame